FIG. 2 illustrates typical Kohler illumination. The effective source (here a lamp LA) is imaged to the aperture stop AS as LAxe2x80x2. Placing film or other photosensitive material at AS will record the intensity distribution. However, the aperture stop is not generally accessible for this sort of diagnostic. For a circularly symmetricsource, the "sgr" value which is defined as:
"sgr"=NAill/NAo
where:
"sgr" partial coherence of effective source
NAill numerical aperture of the cone of radiation defining the effective source.
NAo numerical aperture of the aperture stop as seen from the object side (entrance pupil)
is generally  less than 1. Thus the aperture stop is typically underfilled by the effective source. Control of "sgr" is important for maintaining uniformity of small (xcx9cdiffraction limited resolution) features. A study is known wherein "sgr" variation across a stepper FOV resulted in significant linewidth variations. This study used micro structures (400 nm spaces at various pitches) and indirectly inferred through image simulations the value of "sgr". Such an indirect measurement can only capture one or at most a few parameters characterizing the effective source luminous intensity. A direct method of measurement that separates other effects such as imaging objective aberrations, dose control, photoresist development characteristics and provides a more complete set of information is desirable.
Another effect the effective source has on printed imagery arises from decentration of the effective source with respect to the system exit pupil. This also goes by the name of condenser aberrations or alignment. Condenser alignment can leads to printed image distortion that is a function of defocus. It is important to separate this from distortion which is due to the system imaging objective alone. Many distortion correcting techniques would benefit from a metrology tool that could clearly distinguish that part of the distortion due to condenser setup and that part due to the imaging objective alone.
It is known to use electrical methods (van der Pauw resistors) to ascertain condenser alignment. This technique utilized microstructures at different wafer z positions to infer the z dependent distortion described in reference 6. As such, this technique relied on subtracting out the imaging objective contribution to distortion to arrive at condenser misalignments. A measurement technique that intrinsically and clearly separated imaging objective and condenser effects is desirable.
Other techniques aimed at diagnosing imaging objective behavior, not the effective source distribution include: an insitu interferometer for wavefront determination (ref 8 and ref 9), ref 56 describes an interferometer (noninsitu) for stepper diagnosis, techniques for determining optimal focus, techniques for determining focus and astigmatism only, and general field characterization and qualification techniques.
The current invention is an insitu device that directly measures the luminous intensity (energy per unit solid angle) of the effective source, it""s alignment, shape, and size.
A device, method of measurement and method of data analysis are described for imaging and quantifying in a practice sense the luminous intensity of the effective illumination source of an image system. The device, called a source metrology instrument, produces images and other quantitative measurements of the combined condenser and light source that are taken in situ without any significant alteration of the optical or mechanical set up. As such, the device can be used to monitor and assess the coherence properties of the illumination source with a minimum of interruption to the optical tools productive time. It can be used with photolithographic step and repeat reduction or nonreducing image systems (steppers) scanning image systems, fixed fields step and repeat aberration systems, scanning aberration systems, or any other projection imaging or aberration system.